The present invention provides crucibles for multiple use, as, for example, for a service life of at least five production cycles, in order to achieve an increase in economy in comparison with a conventional SiO2 crucible.
Nitride-bonded silicon carbide (also called NSiC) is a firing aid used primarily as a support structure in the fast firing of porcelain. The production of nitride-bonded silicon carbide is adequately described in the literature. Owing to its chemical composition, NSiC cannot be used directly as a crucible material.
Another firing aid is graphite. This material is also used commercially to produce crucibles. Since, however, carbon undergoes a reaction with silicon to form SiC, graphite too cannot be used directly as a crucible for the melting of silicon. If, for example, silicon is melted directly in a graphite crucible, the graphite of the crucible reacts to form silicon carbide, and, consequently, the crucible is destroyed. Accordingly, it cannot be used directly as a crucible for silicon melting.
From numerous prior attempts, the literature describes that silicon nitride is a suitable material. Silicon nitride (Si3N4) is a material used to manufacture high-grade components for industrial use. Pure silicon nitride powder cannot be compacted to a solid body by temperature treatment without additions (“pure silicon nitride” for the purposes of this specification means an additive-free silicon nitride, in other words, a silicon nitride having no added additives or additive systems). Such a silicon nitride may nevertheless contain minor amounts of impurities.
The known silicon nitride material is “conventional” silicon nitride (Si3N4) with the corresponding additive systems in order to achieve complete compaction. The reference here generally is to what is called a “closed porosity” and to a dense silicon nitride with densities of more than 97%, based on the theoretical density of the sintered silicon nitride, depending on the selected additive system, of >3.2 g/cm3. In order to achieve this degree of density, oxidic additives such as Al2O3, Y2O3 and other oxides of the rare earths are used. Additives from the group of the alkaline earth metal elements as well, such as MgO, for example, have been described. All of these additives result in a compaction process during sintering that is accompanied by a linear contraction of approximately 20%. These additives, however, prevent use as a solar crucible, since Al in particular has an adverse effect on the properties of the solar cells.
WO 2007/148986 from Rec Scanwafer AS describes a mixture of metallic silicon and silicon nitride for producing rectangular crucibles, similarly to RBSN (i.e., reaction-bonded silicon nitride).
DE 10 2005 032 790 A1 describes a base container body, constituting a fired mould or a green mould, which is intended as a container for the melting of non-ferrous metals, and especially of silicon. The container has a coating which comprises at least one of the compounds silicon nitride or silicon dioxide, and the concentration of the silicon dioxide may increase towards the crucible wall. The admixing of silicon dioxide has the advantage of resulting in improved adhesion of the silicon nitride powder on the substrate, such as the wall of an SiO2 crucible. It is thereby generally necessary to admix the powder with an organic constituent for adhesion to the container wall. Prior to use, the container may also be subjected to a temperature treatment, in which case, in contrast to the present invention, there is no compaction of the layer through sintering or reaction. If, however, a temperature treatment takes place, the organic binder is combusted, and a loose powder layer remains. This procedure can occur only with a quartz crucible, since these crucibles are used only once in any case. Moreover, the increasing SiO2 gradient in the crucible direction is an indicator of the use of an SiO2 crucible. Even if the quartz crucible were to be used a number of times, the powder layer would lose its adhesiveness because of the absence of a binder fraction. The layer therefore does not adhere firmly, and consequently, can be used only once.
DE 10 2006 003 820 A1 describes a silicon nitride coating on mouldings of silicon dioxide (SiO2). The silicon nitride layer is produced by application of a layer of Si, followed by subsequent nitriding. In the case of graphite/carbon-based base bodies, coating by means of pure Si is not possible since silicon carbide forms preferentially instead of silicon nitride. Silicon carbide, however, is not a suitable release layer for use for the melting of solar silicon.
Nitride-bonded silicon carbide (i.e., NSiC) is a well-known firing aid. The literature describes the production of a material from nitride-bonded silicon carbide. One production method is the shaping of components by way of slip casting. A mixture of silicon carbide powder and silicon powder is thereby processed to nitride-bonded silicon carbide via a nitriding operation. Optimized production of this material in different grain sizes is described in the literature, through the selection of auxiliaries. NSiC, however, contains a carbidic fraction which is not suitable for the melting of silicon, and particularly not of pure or high-purity silicon. The silicon melt must therefore be protected from direct contact with the NSiC.